1. Field of the Invention
The present invention relates to an oil ring assembly adapted to be used in a piston of an internal combustion engine and, more particularly, to improvements in a three-piece type oil ring adapted to be fitted in an axially narrow oil ring groove formed on the circumference of a piston disposed inside a cylinder of an internal combustion engine.
2. Description of the Prior Art
Recently, because of a growing tendency of internal combustion engines toward higher revolutions, higher outputs and less fuel consumption, shorter pistons and narrower piston rings including oil rings are being required. A three-piece type narrow oil ring assembly is disclosed in Japanese Examined Patent Publication JP-B- 4719653, as a solution to such requirements.
The prior art oil ring assembly disclosed above comprises a spacer expander in the form of a gapped annulus made of section steel having substantially H-shaped cross section, and a pair of upper and lower side rails, each in the form of a gapped flat annulus. The space expander includes an axially protruding outer portion disposed on the outer circumference thereof and facing the inner surface of the cylinder, an axially protruding inner portion disposed on the inner circumference thereof and facing the innermost wall of the oil ring groove, and an axially flat connecting portion extending between the outer and inner protruding portions. The space expander further has a plurality of radial slits formed therein so as to be alternately extending inwardly from the outer circumference, and outwardly from the inner circumference of the spacer expander. The overall axial width of the oil ring was 3 mm or less.
The prior art oil ring assembly, however, has a disadvantage in that, since the side rail supporting upper and lower end surfaces of the outer protruding portion of the spacer expander are extending radially, the upper and lower side rails tend to have adverse rocking motion around either the outer or inner circumferential edge of the upper and lower side rail supporting surfaces with the axially or vertically reciprocating movement of the piston at higher engine revolutions, thereby resulting in a poor side rail attitude stability.
The prior art oil ring assembly also has another disadvantage in that, since the outer circumferential axial surface of the inner protruding portion of the spacer expander is formed parallel to the axial direction, the upper and lower side rails are pressed onto the inner surface of the cylinder by the side rail pressing surface only in the direction perpendicular to the inner surface of the cylinder, the side rails do not come into close contact with the upper and lower end walls of the oil ring groove, thereby resulting in a poor sealing performance.